The invention relates generally to a rotary electric machine having a relatively elongated rotor, and more particularly to cooling the winding of the rotor of the rotary electric machine at multiple points axially.
Rotary electric machines include generators and motors. A generator may be used to convert mechanical energy from a prime mover into electrical energy. A motor performs the opposite function. For simplicity, the discussion hereinbelow is limited primarily to generators.
Generators typically use a rotating magnetic device known as a rotor mounted within a stationary member known as a stator. The rotor is rotatably driven by a prime mover. In an aircraft, a generator may be driven by means of a main engine or an auxiliary power unit (APU) through a gearbox or a constant speed drive (CSD) transmission. The electrical energy produced by the generator illuminates the cabin, powers avionics, heats food, etc. Electrical power requirements typically are greater for newer aircraft as compared with their predecessors because more electrical devices and loads are used; in particular, flight control surfaces are increasingly being actuated by electric power rather than hydraulics.
When the need for electrical power delivered by the generator is relatively large (because more electrical devices are used in newer aircraft as compared with their predecessors), generally the rotating magnetic device is a rotor winding rather than a permanent magnet. The rotor winding becomes an electromagnet when the winding is connected to a current source, and the electromagnet produces a rotating magnetic field of sufficient intensity to generate the relatively large power in a stator winding. The rotor winding generally comprises a plurality of coils of wire, typically copper, wound around a magnetic core, typically cobalt-iron. This arrangement is commonly referred to as xe2x80x9cpoles.xe2x80x9d
Generators produce heat by several mechanisms. Generally, the largest source of heat in a generator is ohmic loss in the rotor and stator windings, that is, the heat produced by the square of the current multiplied by the winding electrical resistance (sometimes known as xe2x80x9ccopper lossxe2x80x9d). Additional heat sources are eddy currents in the stator armature core (xe2x80x9ciron lossxe2x80x9d), bearing friction, and rectifier electrical loss. Generators are typically cooled by means of a fluid (e.g., liquid or a cooled gas), but the motion of the rotor through the fluid itself produces heat by windage (fluid resistance). In relatively small, high-power generators, e.g., aircraft generators, removing the heat from the windings, principally the copper loss, is a design challenge.
Designing a generator involves many tradeoffs. The diameter or the length of the rotor, or both, may be increased when comparatively greater generator output power is required. If the rotor diameter is increased beyond a certain point to increase the power output, centrifugal forces become excessive, turbulence creates excessive drag and heat in the cooling fluid, and the speed of the rotor tips may approach the speed of sound, thereby creating shock waves in the cooling fluid. On the other hand, if the rotor length is increased beyond a certain point to increase the power output, cooling the rotor at the ends only, as is generally done in the prior art, produces a thermal gradient leading to a relatively hot region in the rotor axial mid-region.
Various methods are known to cool a rotor winding. A fluid such as oil may be injected on the rotor winding, or the fluid may bathe the winding. The fluid may be injected radially or flowed axially. In a generator having a rotor and a stator in a common housing, it is usually necessary to cool both the stator winding and the rotor winding. This may be achieved by passing cooling fluid into the rotor and through the rotor winding, and simultaneously jetting fluid from outlets at either end of the rotor onto the stator end winding.
U.S. Pat. No. 5,554,898 describes such an arrangement. Ducts leading from the interior of the rotor to its periphery are provided at opposite ends of the rotor with a flow path therebetween, which passes over the rotor winding. Cooling oil is pumped into the rotor and along the flow path over the rotor winding. Centrifugal force causes the cooling oil to form an annular layer at each end of the rotor. If the rotor is elongated, the middle region axially may not be cooled adequately.
When the rotor is relatively elongated in relation to the rotor diameter, a significant problem is to maintain sufficient heat transfer by means of cooling fluid flow in regions of the rotor winding that are located comparatively far from the rotor ends to prevent those regions of the rotor winding from overheating. The present invention is specifically directed to overcoming this problem.
An object of this invention is to provide a rotary electric machine in which sufficient cooling is maintained at a plurality of regions axially along the rotor winding.
An additional object is to reduce hot spots in the rotor of a high-power generator.
A further object is to provide relatively even cooling along the entire length of a generator rotor without significantly reducing structural integrity.
According to the invention, a rotary electric machine comprises a stator having a plurality of windings and a hollow shaft having an axial end region. The shaft is mounted within the stator for rotation about an axis. The shaft has at least one radial orifice and an inlet for a cooling fluid in the axial end region. A rotor core is integral to and coaxial with the hollow shaft, the rotor core having at least one electromagnetic device formed integral therewith. Each electromagnetic device is disposed in apposition to one of the plurality of stator windings. A generally radial outward passageway through the rotor core has an entry for the cooling fluid at the inner diameter of the rotor core, the entry being aligned with one of the shaft radial orifices. The passageway has an outlet arranged to discharge the cooling fluid onto at least one of the electromagnetic devices.
According to another aspect of the invention, the rotor core further comprises a plurality of laminations, at least one pair of adjacent laminations having periodic slots, wherein the pair of adjacent laminations is sandwiched between a pair of laminations without slots, such that the slots in the adjacent laminations form a continuous, zigzag, generally radial outward passageway.
According to still another aspect of the invention, the pair of adjacent laminations further comprises two laminations having an identical pattern of slots, the two laminations being rotated a number of degrees from each other.
The invention has the benefit that it uses a portion of the flow of the cooling fluid to cool multiple points along the rotor without using openings in the rotor that may weaken the rotor or be prone to clog with contaminants in the cooling fluid.
The above and other objects, features, and advantages of this invention will become apparent when the following description is read in conjunction with the accompanying drawings.